The invention pertains to a push-pull microwave amplifier. More particularly, the invention pertains to a push-pull microwave amplifier which is operable over a broad bandwidth.
To construct a true push-pull microwave amplifier, it is necessary to provide a driver circuit which simulates the behavior of a center-tapped transformer. This is illustrated in FIG. 1 in which an input waveform applied to a primary winding of a center-tapped transformer 10 induces signals across the two halves 14 and 16 of the secondary of the transformer 10 which track each other in magitude but with the opposite sign.
It is noted that, as illustrated in FIG. 2, it is not possible to simulate the behavior of a center-tapped transformer utilizing a simple 180.degree. time delay such as may be achieved by disposing a 180.degree. delay line in one of the output legs of a 3 dB phase splitter. In such a case, it is noted that the output signals are shifted in time from one another. That is, they do not simultaneously track each other in magnitude with opposite signs as is necessary for driving a true push-pull amplifier. To correctly drive a push-pull amplifier, it is desirable that an input signal be split into two halves which are precisely 180.degree. different from one another, and not merely phase shifted with respect to one another, over all frequencies of interest.
In one prior art attempt to accomplish the desired function, as described in the article "A MIC Push-Pull FET Amplifier", Westinghouse Electric Corporation, Advanced Technology Laboratories, Baltimore, Md. and in U.S. Pat. No. 4,097,814 issued June 27, 1978 to Cohn, an MIC hybrid junction which is similar in operation to a well-known waveguide "magic tee" is known in which the junction is fabricated with a dielectric substrate with opposite planar faces metallized in a specific pattern. One face is completely metallized except for a "tee" shaped slot while the opposite face contains only a single microstrip line.
Another prior art approach, as described in the article "Coplanar Balun Circuits for GaAs FET High Power Push-Pull Amplifiers", 1973 IEEE G-MTT International Microwave Symposium, Boulder, Colo., June 4-6, 1973, pp. 309-311, a Balun circuit is utilized to convert a "hot" conductor/ground transmission line configuration to a two-conductor transmission line where both conductors are isolated from ground and currents flow equally in each conductor 180.degree. out of phase with one another. The Balun circuit in that article is fabricated from three coplanar conductors supported by a dielectric substrate. Three parallel strip transmission lines extend from the center conductor into sides of an input coaxial line. At one-quarter wavelength from the juncture between the coaxial line and the three parallel strip lines, a short is made between the center conductor and one of the conductors connected to the ground side of the input coaxial line. The balanced line then continues from the other of the ground lines and the center conductor.
Both of these approaches suffer from the disadvantage that only narrow-band operation is achievable. Also, both of these approaches require a special form of microwave transmission line (a slotted line in the first-mentioned reference and a coplanar waveguide in the second). Moreover, neither of these techniques is usable in a "microstrip" circuit, the type of microwave integrated circuit in most common usage at the present time.
It is thus a primary object of the present invention to provide a microwave amplifier circuit in which an input signal is split into two halves which are 180.degree. out of phase with one another over a wide bandwidth, wherein the behavior of a center-tapped transformer is accurately simulated.
It is a further object of the invention to provide such a microwave push-pull amplifier circuit in which only standard microwave "microstrip" circuit techniques are utilized.